1. Field of the Invention
The present invention relates to methods of inspection and production of a mask used in a lithography step in production of a semiconductor device etc. and to a mask defect inspection system.
2. Description of the Related Art
Along with the miniaturization of semiconductor devices, the pattern size on a photomask used for photolithography has become increasingly smaller. Up to now, in photolithography, accuracy of formation of fine patterns has been improved by shortening the exposure wavelength. However, in recent years, transfer patterns have become shorter than the exposure wavelength, so super-resolution techniques have been broadly used to extend the life of photomasks at the same exposure wavelength.
Super-resolution techniques are roughly classified into two types: ones conducted at an exposure system (stepper or scanner) side and ones conducted on a mask side. The former techniques are called “modified illumination” and change the method of illumination to improve the resolution. The orbicular zone illumination being widely used at present can be mentioned as an example.
On the other hand, a half-tone type phase shift mask, a Levenson phase shift mask, or other mask is used for the latter techniques. A half-tone type phase shift mask uses a semi-transparent film for its light-blocking regions and generates portions of a zero light intensity by inverting the phase of light passing through light-passing regions and the phase of light passing through the light-blocking regions. A half-tone type phase shift mask is sometimes used together with modified illumination.
A Levenson phase shift mask provides portions differing in optical path lengths (phase shifters) in the non-light blocking regions (light-passing regions). For example, parts of the substrate are removed at non-light-block regions so as to give a phase difference to the light according to where it passes through the mask. Due to this, the resolution of patterns is improved.
According to the super-resolution techniques, miniaturization of patterns becomes possible without shortening the exposure wavelength. However, compared with when miniaturizing patterns by shortening the exposure wavelength, in general, in super-resolution techniques, line width errors, defects, etc. on the mask have a greater effect on the transfer patterns.
Further, when forming a dense pattern on the phase shift mask, the amount of the light passing through the mask decreases and sufficient contrast cannot be obtained, so the effect of defects becomes greater. As opposed to this, when forming a sparse pattern, the amount of light passing through the vicinity of defects increases compared with a dense pattern, so that the effect of defects on the transfer becomes relatively smaller.
This phenomenon of the magnitude of the effect of defects on transfer changing depending on the pattern density is also observed in conventional photolithography not using super-resolution techniques. The use of super-resolution techniques however increases the difference between the effects of dense and sparse patterns. Up to now, defects have been determined by comparison with a flat acceptable defect size determined without regard to the pattern density.
Summarizing the problems to be solved by the invention, when using super-resolution techniques, if determining defects based on a flat acceptable defect size, there is a possibility of defects which might actually affect transfer being allowed in dense portions of a pattern. Conversely, there is a possibility of defect which actually have no effect on transfer not being allowed in sparse portions of a pattern. Therefore, with the conventional method of detection of defects based on a flat acceptable defect size, it becomes difficult to determine defects.
A defect is usually repaired by removing the corresponding part of the light-blocking film by a focused ion beam (FIB) or by depositing additional material by the FIB in the presence of an organic gas. Generally, the smaller the defect, the more difficult it is to repair the defect with a high accuracy. Therefore, when setting a flat acceptable defect size, if the acceptable defect size is made smaller and the number of defects increases, the time required for precise repair of all the defects becomes longer. Due to this, the throughput of the production of masks is lowered.
If trying to shorten the time required for repairing defects, the problem arises of a drop in the probability of repair of defects at a defect repair system. Namely, the yield in the production of masks is lowered. Consequently, it has become important to select defects requiring repair instead of repairing all the defects detected at inspections without selection.